Optical arrangement of autofocus elements for use with immersion lithography

ABSTRACT

A lithographic projection apparatus includes an optical element through which a substrate is exposed with an exposure beam. A space between the optical element and the substrate is filled with liquid during the exposure. A gap is formed between a member and a surface of the optical element through which the exposure beam does not pass. The liquid is supplied to the gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of U.S. patent application Ser. No. 12/457,742 filed Jun. 19, 2009, which in turn is a Divisional of U.S. patent application Ser. No. 11/606,914 filed Dec. 1, 2006 (now U.S. Pat. No. 7,570,431), which in turn is a Divisional of U.S. patent application Ser. No. 11/234,279 filed Sep. 26, 2005 (now U.S. Pat. No. 7,414,794), which in turn is a Continuation of International Application No. PCT/US2004/011287 filed Apr. 12, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/464,392 filed Apr. 17, 2003. The disclosures of the above-identified applications are incorporated by reference herein in their entireties.

BACKGROUND

This invention relates to an optical arrangement of autofocus elements for use with immersion lithography.

In semiconductor lithography systems in use today, automatic focusing and leveling (AF/AL) is typically accomplished by passing a low angle of incidence optical beam onto the surface of a silicon wafer and detecting its properties after subsequent reflection from the wafer surface. The wafer height is determined by optical and electrical processing of the reflected light beam. This beam passes under the last element of the projection lens. The source and receiver optics are typically mounted to a stable part of the system, close to the projection optics mounting position.

In immersion lithography, a liquid such as water fills the space between the last surface of the projection lens and the wafer. At the edge of the water, typically at the edge of the lens or supported structure near the edge of the lens, the liquid-air boundary is not well defined and is changing rapidly. It is not possible to transmit an AF/AL beam through this interface without substantial disruption and subsequent loss of signal, and hence performance.

It is therefore a general object of this invention to provide a way to introduce AF/AL beams into the liquid layer without such disruption so as to preserve the optical accuracy and stability required.

More specifically, it is an object of this invention to provide an apparatus and a method for allowing AF/AL light beams to be used as in conventional lithography without the disrupting influence of the liquid immersion boundary at the edge of the lens.

SUMMARY

Autofocus units according to this invention are for an immersion lithography apparatus that may be described generally as comprising a reticle stage arranged to retain a reticle, a working stage arranged to retain a workpiece having a target surface, an optical system including an illumination source and an optical element such as a lens positioned opposite and above the workpiece for projecting an image pattern of the reticle onto the workpiece by radiation from the illumination source, and a fluid-supplying device for providing a fluid into the space defined between the optical element and the workpiece such that the fluid contacts both the optical element and the target surface of the workpiece. The optical element positioned opposite to the workpiece may be treated as a component of the autofocus unit itself which may be characterized as further comprising an autofocus light source serving to project a light beam obliquely at a specified angle such that this light beam passes through the fluid and is reflected by the target surface of the workpiece at a specified reflection position that is below the optical element, and a receiver for receiving and analyzing the light beam reflected by the target surface. Correction lenses preferably may be disposed on the optical path of the light beam projected from the autofocus light source for correcting propagation of the light beam.

As an alternative embodiment, the optical element opposite the workpiece may be cut on its two mutually opposite sides, and optically transparent wedge-shaped elements may be placed under these cuts such that the light beam from the autofocus light source will pass through them as it is passed through the fluid to be reflected on the target surface of the workpiece and to reach the receiver without passing through the optical element at all. In order to cope with the potential problem of bubbles that may be formed due to the gap between the wedge element and the optical element, the gap may be filled with a suitable material, made sufficiently narrow such as less than 2.0 mm such that capillary forces will keep the gap filled with the fluid, or provided with means for supplying a small suction to cause the fluid to move up through the gap or to supply the fluid such that the gap can be kept filled. The boundary surface through which the light beam from the autofocus light source is refracted into the fluid from the interior of the wedge element need not be parallel to the target surface of the workpiece, but may be appropriately sloped, depending on the indices of refraction of the materials that affect the design of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanying drawings of exemplary embodiments in which like reference numerals designate like elements, and in which:

FIG. 1 is a schematic cross-sectional view of an immersion lithography apparatus that incorporates the invention;

FIG. 2 is a process flow diagram illustrating an exemplary process by which semiconductor devices are fabricated using the apparatus shown in FIG. 1 according to the invention;

FIG. 3 is a flowchart of the wafer processing step shown in FIG. 2 in the case of fabricating semiconductor devices according to the invention;

FIG. 4 is a schematic side cross-sectional view of an autofocus unit embodying this invention;

FIG. 5 is a schematic side cross-sectional view of another autofocus unit embodying this invention;

FIG. 6 is an enlarged view of a circled portion 6 of FIG. 5; and

FIGS. 7, 8 and 9 are schematic side cross-sectional views of portions of other autofocus units embodying this invention according to different embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows the general structure of an immersion lithography apparatus 100 that may incorporate the optical arrangement of autofocus elements embodying this invention.

As shown in FIG. 1, the immersion lithography apparatus 100 comprises an illuminator optical unit 1 including a light source such as a KrF excimer laser unit, an optical integrator (or homogenizer) and a lens and serving to emit pulsed ultraviolet light IL with wavelength 248 nm to be made incident to a pattern on a reticle R. The pattern on the reticle R is projected onto a wafer W coated with a photoresist at a specified magnification (such as ¼ or ⅕) through a telecentric light projection unit PL. The pulsed light IL may alternatively be ArF excimer laser light with wavelength 193 nm, F₂ laser light with wavelength 157 nm or the i-line of a mercury lamp with wavelength 365 nm. In what follows, the coordinate system with X-, Y- and Z-axes as shown in FIG. 1 is referenced to explain the directions in describing the structure and functions of the lithography apparatus 100. For the convenience of disclosure and description, the light projection unit PL is illustrated in FIG. 1 only by way of its last-stage optical element (such as a lens) 4 disposed opposite to the wafer W and a cylindrical housing 3 containing all others of its components.

The reticle R is supported on a reticle stage RST incorporating a mechanism for moving the reticle R by some amount in the X-direction, the Y-direction and the rotary direction around the Z-axis. The two-dimensional position and orientation of the reticle R on the reticle stage RST are detected by a laser interferometer (not shown) in real time and the positioning of the reticle R is affected by a main control unit 14 on the basis of the detection thus made.

The wafer W is set on a wafer holder (not shown) on a Z-stage 9 for controlling the focusing position (along the Z-axis) and the sloping angle of the wafer W. The Z-stage 9 is affixed to an XY-stage 10 adapted to move in the XY-plane substantially parallel to the image-forming surface of the light projection unit PL. The XY-stage 10 is set on a base 11. Thus, the Z-stage 9 serves to match the wafer surface with the image surface of the light projection unit PL by adjusting the focusing position (along the Z-axis) and the sloping angle of the wafer W by the auto-focusing and auto-leveling method, and the XY-stage 10 serves to adjust the position of the wafer W in the X-direction and the Y-direction.

The two-dimensional position and orientation of the Z-stage 9 (and hence also of the wafer W) are monitored in real time by another laser interferometer 13 with reference to a mobile mirror 12 affixed to the Z-stage 9. Control data based on the results of this monitoring are transmitted from the main control unit 14 to a stage-driving unit 15 adapted to control the motions of the Z-stage 9 and the XY-stage 10 according to the received control data. At the time of an exposure, the projection light is made to sequentially move from one to another of different exposure positions on the wafer W according to the pattern on the reticle R in a step-and-repeat routine.

The lithography apparatus 100 described with reference to FIG. 1 is an immersion lithography apparatus and is hence adapted to have a liquid (or the “immersion liquid”) 7 of a specified kind such as water filling the space between the surface of the wafer W and the lower surface of the last-stage optical element 4 of the light projection unit PL at least while the pattern image of the reticle R is being copied onto the wafer W.

The last-stage optical element 4 of the light projection unit PL is affixed to the cylindrical housing 3. In an optional embodiment, the last-stage optical element 4 may be made removable for cleaning or maintenance.

The liquid 7 is supplied from a liquid supply unit 5 that may comprise a tank, a pressure pump and a temperature regulator (not individually shown) to the space above the wafer W under a temperature-regulated condition and is collected by a liquid recovery unit 6. The temperature of the liquid 7 is regulated to be approximately the same as the temperature inside the chamber in which the lithography apparatus 100 itself is disposed. Numeral 21 indicates source nozzles through which the liquid 7 is supplied from the supply unit 5. Numeral 23 indicates recovery nozzles through which the liquid 7 is collected into the recovery unit 6. The structure described above with reference to FIG. 1 is not intended to limit the scope of the immersion lithography apparatus to which the methods and devices of the invention are applicable. In other words, autofocus units of the invention may be incorporated into immersion lithography apparatus of many different kinds. In particular, the numbers and arrangements of the source and recovery nozzles 21 and 23 around the light projection unit PL may be designed in a variety of ways for establishing a smooth flow and quick recovery of the immersion liquid 7.

FIG. 4 shows an autofocus unit 50 (not shown in FIG. 1) according to this invention which may be incorporated into an immersion lithography system such as shown at 100 in FIG. 1, but the invention is not intended to be limited by the specific type of the system into which it is incorporated. In this example, the last-stage optical element 4 of the light projection unit PL is a hemispherically shaped projection lens with its planar surface facing downward opposite to the upper surface (the “target surface”) of the wafer W, with a space left in between. An autofocus light source 51 is arranged such that its AF/AL light beam 54, emitted obliquely with respect to the target surface of the wafer W, passes through a lower peripheral part of this lens 4 and then is refracted into the immersion liquid 7 so as to be reflected by the target surface of the wafer W at a specified reflection position 55. A receiver 52 for receiving and analyzing the reflected AF/AL light beam 54 is appropriately positioned on the opposite side of the light projection unit PL. Numerals 53 each indicate what may be referred to as a correction lens disposed on the path of the AF/AL light beam 54 for correcting light propagation. Since the interface between the lens 4 and the liquid 7 is well defined and essentially free of bubbles, the light beams are unimpeded and can provide good signals to maintain high accuracy. In FIG. 4, broken lines indicate the exposure light cone, or the boundary of the exposure region.

FIG. 5 shows another autofocus unit 60 according to another embodiment of the invention. Its components that are similar to those described above with reference to FIG. 5 are indicated by the same numerals. The unit 60 shown in FIG. 5 is characterized as having the lower surface of the last-stage optical element 4 of the light projection unit PL cut in two places facing respectively the autofocus light source 51 and the receiver 52. The cut surfaces preferably may be flat, as shown in FIG. 5, and the last-stage optical element 4 is still functionally and essentially a hemispherical lens. Optically transparent parts, referred to as wedge elements 61 and 62, are placed on both sides of the lens 4 under these cut surfaces, the element 61 being on the side of the autofocus light source 51 and the element 62 being on the side of the receiver 52. The cuts and the wedge elements 61 and 62 are designed so that the AF/AL light beam 54 from the autofocus light source 51 will pass through the wedge element 61 and be refracted into the immersion liquid 7 without passing through the lens 4 and, after being reflected by the target surface of the wafer W at the reflection position 55, will be refracted into the wedge element 62 and received by the receiver 52 again without passing through the lens 4. This embodiment is advantageous because the wedge elements can be made of a different material from the lens element 4, such as optical glass.

The lower interface between the wedge elements 61 and 62 and the lens 4 is important from the points of view of correct optical performance and generation of bubbles in the immersion liquid 7. With reference to FIG. 6, which shows in more detail the portion of the wedge element 61 in a close proximity of the lens 4, the gap D therebetween is a potential source of air bubbles, which may be entrained under the lens 4, adversely affecting its optical performance.

One of the solutions to this problem is to fill the gap with a suitable material or to press the wedge element 61 into contact with the lens 4 such that the gap D becomes effectively zero and therefore does not perturb the liquid interface. Another solution is to keep D approximately equal to or less than 2.0 mm such that capillary forces cause the liquid 7 to fill the gap and keep it filled even while the wafer W is moved under the lens 4. A third solution is to supply a small suction to cause the liquid 7 to move up inside the gap D and to prevent air from moving downward, as shown in FIG. 7 in which numeral 70 indicates an air pump for providing the suction. FIG. 8 shows still another solution whereby a source 72 of the liquid 7 is supplied above the opening of the gap D to keep the gap D filled with the liquid 7.

The invention has been described above with reference to only a limited number of arrangements, but they are not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of the invention. The shape of the wedge elements 61 and 62, for example, need not be as described above with reference to FIG. 6. Depending, for example, upon the desired angle of incidence of the AF/AL light beam 54 relative to the indices of refraction of the immersion liquid 7 and the material of the wedge element 61, it may be advantageous, as shown in FIG. 9, to provide the wedge element 61 with a sloped surface portion 64 such that the AF/AL light beam 54 passing through the wedge element 61 will be refracted into the immersion liquid 7, not necessarily through a horizontal boundary surface as shown in FIG. 6, but through this appropriately sloped surface portion 64. This will provide flexibility in the design of the arrangement embodying this invention.

FIG. 2 is referenced next to describe a process for fabricating a semiconductor device by using an immersion lithography apparatus incorporating a liquid jet and recovery system embodying this invention. In step 301 the device's function and performance characteristics are designed. Next, in step 302, a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step 303, a wafer is made from a silicon material. The mask pattern designed in step 302 is exposed onto the wafer from step 303 in step 304 by a photolithography system such as the systems described above. In step 305 the semiconductor device is assembled (including the dicing process, bonding process and packaging process), then finally the device is inspected in step 306.

FIG. 3 illustrates a detailed flowchart example of the above-mentioned step 304 in the case of fabricating semiconductor devices. In step 311 (oxidation step), the wafer surface is oxidized. In step 312 (CVD step), an insulation film is formed on the wafer surface. In step 313 (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step 314 (ion implantation step), ions are implanted in the wafer. The aforementioned steps 311-314 form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements.

At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, initially, in step 315 (photoresist formation step), photoresist is applied to a wafer. Next, in step 316 (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) onto a wafer. Then, in step 317 (developing step), the exposed wafer is developed, and in step 318 (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step 319 (photoresist removal step), unnecessary photoresist remaining after etching is removed. Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps.

While a lithography system of this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and various substitute equivalents that fall within the scope of this invention. There are many alternative ways of implementing the methods and apparatus of the invention. 

1. A lithographic projection apparatus comprising: an optical element through which a substrate is exposed with an exposure beam, a space between the optical element and the substrate being filled with liquid during the exposure; a member disposed adjacent to a surface of the optical element through which the exposure beam does not pass; and a gap formed between the member and the surface of the optical element through which the exposure beam does not pass, wherein liquid is supplied to the gap, and wherein the optical element and the member are configured such that the substrate moves relative to the optical element and the member during an exposure operation in which the exposure beam is directed onto the substrate.
 2. The lithographic projection apparatus of claim 1, wherein a first surface of the member faces the surface of the optical element through which the exposure beam does not pass, and a second surface of the member faces the substrate.
 3. The lithographic projection apparatus of claim 2, wherein the first surface of the member contacts the liquid in the gap, and the second surface of the member contacts the liquid on the substrate.
 4. The lithographic projection apparatus of claim 3, wherein the second surface of the member is substantially coplanar with a surface of the optical element through which the exposure beam passes.
 5. The lithographic projection apparatus of claim 2, wherein the second surface of the member is substantially coplanar with a surface of the optical element through which the exposure beam passes.
 6. The lithographic projection apparatus of claim 2, wherein the liquid is allowed to fill a space between the substrate and the second surface of the member during exposure of the substrate.
 7. The lithographic projection apparatus of claim 1, wherein the gap is formed between a side surface of the optical element and the member.
 8. The lithographic projection apparatus of claim 1, wherein the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion.
 9. The lithographic projection apparatus of claim 1, wherein the gap slopes away from an optical axis of the optical element as the gap becomes more distant from the substrate.
 10. The lithographic projection apparatus of claim 1, wherein a surface of the substrate is positioned opposite to an end surface of the optical element through which the exposure beam passes, and the liquid covers only a part of the surface of the substrate during the exposure.
 11. The lithographic projection apparatus of claim 10, wherein the liquid is retained between the end surface of the optical element and the substrate, and between a lower surface of the member and the substrate.
 12. The lithographic projection apparatus of claim 10, wherein the member has a lower surface adjacent to the end surface of the optical element, and a portion of the liquid on the substrate is retained between the lower surface of the member and the substrate.
 13. The lithographic projection apparatus of claim 1, wherein a surface of the member that faces the substrate is substantially coplanar with an end surface of the optical element through which the exposure beam passes.
 14. The lithographic projection apparatus of claim 1, wherein the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, and the liquid is supplied to the space from the first end portion of the gap.
 15. The lithographic projection apparatus of claim 14, wherein a source of the liquid is coupled to the second end portion of the gap.
 16. A device manufacturing method comprising: exposing a substrate using the apparatus defined in claim 1, and developing the exposed substrate.
 17. A lithographic projection method comprising: providing a substrate so that the substrate faces an optical element and a member disposed adjacent to the optical element; filling a space between the optical element and the substrate with a liquid through which an exposure beam is directed onto the substrate; supplying liquid to a gap between the member and a surface of the optical element through which the exposure beam does not pass; and moving the substrate relative to the optical element and the member during an exposure operation in which the exposure beam is directed onto the substrate.
 18. The method of claim 17, wherein a first surface of the member faces the surface of the optical element through which the exposure beam does not pass, and a second surface of the member faces the substrate.
 19. The method of claim 18, wherein the first surface of the member contacts the liquid in the gap, and the second surface of the member contacts the liquid on the substrate.
 20. The method of claim 19, wherein the second surface of the member is substantially coplanar with a surface of the optical element through which the exposure beam passes.
 21. The method of claim 18, wherein the second surface of the member is substantially coplanar with a surface of the optical element through which the exposure beam passes.
 22. The method of claim 18, wherein the liquid is allowed to fill a space between the substrate and the second surface of the member during exposure of the substrate.
 23. The method of claim 17, wherein the gap is formed between a side surface of the optical element and the member.
 24. The method of claim 17, wherein the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion.
 25. The method of claim 17, wherein the gap slopes away from an optical axis of the optical element as the gap becomes more distant from the substrate.
 26. The method of claim 17, wherein a surface of the substrate is positioned opposite to an end surface of the optical element through which the exposure beam passes, and the liquid covers only a part of the surface of the substrate during the exposure.
 27. The method of claim 26, wherein the liquid is retained between the end surface of the optical element and the substrate, and between a lower surface of the member and the substrate.
 28. The method of claim 26, wherein the member has a lower surface adjacent to the end surface of the optical element, and a portion of the liquid on the substrate is retained between the lower surface of the member and the substrate.
 29. The method of claim 17, wherein a surface of the member that faces the substrate is substantially coplanar with an end surface of the optical element through which the exposure beam passes.
 30. The method of claim 17, wherein the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, and wherein the liquid is supplied to the space from the first end portion of the gap.
 31. The method of claim 30, wherein a source of the liquid is coupled to the second end portion of the gap.
 32. A device manufacturing method comprising: exposing a substrate using the method defined in claim 17, and developing the exposed substrate.
 33. A lithographic projection apparatus comprising: an optical element through which a substrate is exposed with an exposure beam, a space between the optical element and the substrate being filled with liquid during the exposure; and a gap formed between a member and a surface of the optical element through which the exposure beam does not pass, wherein liquid is supplied to the gap, and the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion.
 34. A lithographic projection apparatus comprising: an optical element through which a substrate is exposed with an exposure beam, a space between the optical element and the substrate being filled with liquid during the exposure; and a gap formed between a member and a surface of the optical element through which the exposure beam does not pass, wherein liquid is supplied to the gap, and the gap slopes away from an optical axis of the optical element as the gap becomes more distant from the substrate.
 35. A lithographic projection apparatus comprising: an optical element through which a substrate is exposed with an exposure beam, a space between the optical element and the substrate being filled with liquid during the exposure; and a gap formed between a member and a surface of the optical element through which the exposure beam does not pass, wherein liquid is supplied to the gap, and a surface of the substrate is positioned opposite to an end surface of the optical element through which the exposure beam passes, and the liquid covers only a part of the surface of the substrate during the exposure.
 36. The lithographic projection apparatus of claim 35, wherein the liquid is retained between the end surface of the optical element and the substrate, and between a lower surface of the member and the substrate.
 37. The lithographic projection apparatus of claim 35, wherein the member has a lower surface adjacent to the end surface of the optical element, and a portion of the liquid on the substrate is retained between the lower surface of the member and the substrate.
 38. A lithographic projection method comprising: providing a substrate so that the substrate faces an optical element and a member disposed adjacent to the optical element; filling a space between the optical element and the substrate with a liquid through which an exposure beam is directed onto the substrate; and supplying liquid to a gap between the member and a surface of the optical element through which the exposure beam does not pass, wherein the gap includes first and second end portions located at opposite ends of the gap, the first end portion disposed adjacent to the substrate, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion.
 39. A lithographic projection method comprising: providing a substrate so that the substrate faces an optical element and a member disposed adjacent to the optical element; filling a space between the optical element and the substrate with a liquid through which an exposure beam is directed onto the substrate; and supplying liquid to a gap between the member and a surface of the optical element through which the exposure beam does not pass, wherein the gap slopes away from an optical axis of the optical element as the gap becomes more distant from the substrate.
 40. A lithographic projection method comprising: providing a substrate so that the substrate faces an optical element and a member disposed adjacent to the optical element; filling a space between the optical element and the substrate with a liquid through which an exposure beam is directed onto the substrate; and supplying liquid to a gap between the member and a surface of the optical element through which the exposure beam does not pass, wherein a surface of the substrate is positioned opposite to an end surface of the optical element through which the exposure beam passes, and the liquid covers only a part of the surface of the substrate during the exposure.
 41. The method of claim 40, wherein the liquid is retained between the end surface of the optical element and the substrate, and between a lower surface of the member and the substrate.
 42. The method of claim 40, wherein the member has a lower surface adjacent to the end surface of the optical element, and a portion of the liquid on the substrate is retained between the lower surface of the member and the substrate.
 43. A lithographic projection apparatus comprising: an optical element through which a substrate is exposed with an exposure beam, a space between the optical element and the substrate being filled with liquid during the exposure; a member disposed adjacent to a surface of the optical element through which the exposure beam does not pass; and a gap formed between the member and the surface of the optical element, the gap including first and second end portions located at opposite ends of the gap, the first end portion located closer to the substrate than the second end portion during the exposure, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion, wherein the liquid is supplied to the gap.
 44. A lithographic projection method comprising: providing a substrate so that the substrate faces an optical element and a member disposed adjacent to the optical element; filling a space between the optical element and the substrate with liquid through which an exposure beam is directed onto the substrate; and supplying liquid to a gap between the member and a surface of the optical element through which the exposure beam does not pass, the gap including first and second end portions located at opposite ends of the gap, the first end portion located closer to the substrate than the second end portion during the exposure, the first end portion located closer to an optical axis of the optical element than the second end portion such that the gap slants away from the optical axis from the first end portion toward the second end portion. 